This invention relates to a safe, non-resealable vent closure for galvanic cells, such as nonaqueous liquid oxyhalide cells, and more particularly to an improved vent liner and cell cover for such cells.
Reliable, long service life cells or batteries have been developed for portable electrically powered devices such as tape recorders, playback machines, radio transmitters and receivers. Electrochemical cell systems for such devices provide a long service life by utilizing highly reactive anode materials such as lithium, sodium and the like, in conjunction with high energy density nonaqueous liquid cathode materials and suitable salts, often referred to as cathode-electrolytes.
Galvanic cells typically are sealed to prevent loss of electrolyte by leakage. This is especially important in the case of nonaqueous liquid cathode cells, which typically employ highly reactive oxyhalide or halide cathode-electrolytes. Any escape of such liquids, or their reaction products, could cause damage to the device employing the cell, or to the surface of a compartment or shelf where the cell is stored.
On the other hand, certain operating conditions can cause the internal pressure of such liquid cathode cells to markedly increase. This pressure can be caused by external sources, such as fire, or internal sources, such as heat generated during charging. In certain situations, the anode can melt and react directly with the liquid cathode in a vigorous, energy-releasing reaction. In the case of other galvanic cells, such as alkaline-zinc cells, carbon-zinc cells, etc., such cells may generate large quantities of gas under certain conditions of use. Thus, if any of the foregoing cells were permanently sealed, the build up of internal pressure within the cell could cause the cell container to leak, bulge or even rupture, which can cause property and/or bodily damage.
It is therefore necessary to provide a vent for galvanic cells that is designed to remain sealed during normal operating conditions which the cell may encounter, but which will open when the pressure within the cell substantially increases. In the case of liquid cathode cells employing, for example, a lithium anode, the vent must open before the lithium melts and reacts directly with the liquid cathode. Upon venting, most of the liquid cathode material is removed and is thus unavailable for reaction with the anode.
One type of vent assembly previously used for lithium-oxyhalide cells comprises a vent liner of a material such as polytetrafluoroethylene inserted into an orifice in a cell cover, with a seal member such as a glass ball forced into the orifice of the liner to seal the cell. Upon build up of a predetermined pressure within the cell, the seal member will be at least partially expelled from the liner orifice, thereby forming a permanent vent to the atmosphere. In manufacturing such a vent assembly, an orifice typically is formed by punching a hole in the cell cover. Thereafter, the liner is inserted in the cell cover orifice. Preferably, the liner is flanged on its upper edge so that it will be accurately positioned upon insertion into the cell cover orifice. After insertion, the flange abuts the upper surface of the cover, and a portion of the liner may extend beyond the bottom surface of the cover into the cell interior. Since the punching operation leaves a rough edge at the intersection of the walls of the orifice and the top of the cover, which could detrimentally score the liner, the liner is not forced or press-fitted into the cell cover orifice to provide a tight fit.
Punching out the hole in the cell cover to create an orifice into which the vent liner is inserted results in the orifice being outwardly tapered toward the bottom of the cell cover. As a result, a crevasse will exist between the cell cover and the vent liner. This crevasse will fill with cathode electrolyte fluid when the cell is filled. In consequence, an undesirable electrochemical cell system is created between the lithium in the cell, and oxygen or water vapor, or both, present in the atmosphere directly outside the cell cover. The lithium is oxidized according to: EQU Li.fwdarw.Li.sup.30 +e.sup.31
The lithium ions will diffuse out of the cell through the electrolyte contained in the crevasse and at the vent liner/cover interface, whereupon atmospheric water and oxygen are reduced according to: EQU 1/2 O.sub.2 +H.sub.2 O+2Li.sup.+ +2e.sup.- .fwdarw.2LiOH EQU 2LiOH+CO.sub.2 .fwdarw.Li2CO3 +1/2 H.sub.2.
One or more of these products, which are formed on the exterior of the cell cover, can be extremely corrosive, and in combination with the driving potential of the undesirable electrochemical cell system, could cause leakage of the cell to accelerate with time and could also cause short-circuiting of the cell.